“All in all, this was a pretty shameful day for Washington,” Obama said, accusing the gun lobby of lying about the bill.

The vote was 54-46, with supporters falling six votes short of the required 60-vote threshold.

The failure of the background check proposal authored by Sens. Joe Manchin, D-W.Va., and Pat Toomey, R-Pa., now imperils the entire legislation. The proposal would have expanded background checks to gun shows and Internet sales while exempting personal transactions. The amendment was aimed at winning over reluctant conservatives, who were opposed to the more stringent background check plan in the existing bill.

It’s unclear where supporters will go from here. They could try to vote again, or craft an alternative piece of legislation.

Obama vowed to press on, saying the vote was “just round one,” while decrying those he claimed “caved” to political pressure.

“The gun lobby and its allies willfully lied about the bill,” Obama said. He said the claims “upset” some gun owners who in turn “intimidated” senators.

“There were no coherent arguments as to why we couldn’t do this. It came down to politics,” he said.

In a statement, Manchin said that while he is disappointed in the outcome of today’s vote, that “this is not the end of the debate.”

Opponents, which included a few Democrats, voiced concern that the proposal would infringe on Second Amendment rights by imposing a burden on those buying and selling guns. They claimed the proposed system would not have prevented Newtown, and would not stop criminals. They also voiced concern about the possibility that the expanded system could lead to a gun registry, though the amendment language prohibits this.

The NRA said in a statement that the amendment “would have criminalized certain private transfers of firearms between honest citizens, requiring lifelong friends, neighbors and some family members to get federal government permission to exercise a fundamental right or face prosecution.”

But Senate Majority Leader Harry Reid and other Democrats hammered Republicans for not voting in support and vowed to press forward.

“I want everyone to understand this is just the beginning, not the end,” said Reid. “Today, the brand of the Republican Party has become more out of step, more extreme. And that’s saying something.”

Four Republicans voted for the amendment, but five Democrats voted against it. One of those Democrats was Reid — who only switched his vote to oppose it because doing so allows Democrats to call up the measure again. Other Democrats who voted against the measure for non-procedural reasons were Sen. Mark Pryor of Arkansas, Sen. Mark Begich of Alaska, Sen. Heidi Heitkamp of North Dakota and Sen. Max Baucus of Montana.

The Obama administration has made the package, written in the wake of the Newtown school mass shooting, a top priority and along with its allies had applied heavy pressure to wavering lawmakers. Vice President Biden presided over the vote Wednesday.

Lawmakers proceeded to vote on a string of other amendments Wednesday, including a proposed ban on assault weapons and a ban on high-capacity magazines, which were defeated as well.

In the run-up to Wednesday’s vote, Democratic leaders gave ever-changing assessments of where support stood.

Biden said Tuesday that Democrats would get the 60 votes, but then said later in the day that it could come down to one or two senators.

Manchin acknowledged early Wednesday that the bill was having trouble, but then released a statement saying he remained “optimistic and hopeful.”

Opponents needed just 41 of the Senate’s 100 votes to derail the Manchin-Toomey background check plan.

Thirty-one senators voted last week to completely block debate on overall gun legislation. Since last week, enough lawmakers who voted to allow debate switched to oppose Manchin-Toomey, in turn defeating the amendment.

“I believe very strongly that our current background check system needs strengthening and improving, particularly in areas that could keep guns out of the hands of felons and the mentally ill. At the same time, I cannot support legislation that infringes upon the constitutional right to keep and bear arms,” Sen. Dean Heller, R-Nev., one of those opposed, said in a statement.

Sen. Mike Lee, R-Utah, echoed Heller in a statement released following the vote saying “Following the tragedy at Sandy Hook, we all wanted to find answers that would reduce crime and prevent the next senseless act of violence. Unfortunately, the Senate did not consider any proposals that would achieve these objectives.”

Lee also said he agreed with the president that the debate was not over and hoped that they could “discuss the problems that lead to these violent acts” and work on solutions that actually address them.

Only four Republican senators committed to voting for the amendment ahead of time. The last was Sen. John McCain, R-Ariz., who announced his support Wednesday afternoon. The other three were Toomey, Sen. Mark Kirk of Illinois and Sen. Susan Collins of Maine.

The Senate gun bill would extend background checks to nearly all gun purchases, toughen penalties against illegal gun trafficking and add small sums to school safety programs.

Perhaps helping explain Democrats’ problems, an AP-GfK poll this month showed that 49 percent of Americans support stricter gun laws. That was down from 58 percent who said so in January — a month after the December killings of 20 children and six aides at a Newtown, Conn., elementary school propelled gun violence into a national issue. Family members of Newtown victims, some tearing up after the vote, also criticized the Senate for the amendment’s failure Wednesday.

The Senate held eight other votes Wednesday besides the one on background checks, all of them amendments to the broad gun control measure.

They included Democratic proposals to ban assault weapons and high-capacity ammunition magazines, which are expected to lose; a Republican proposal requiring states to honor other states’ permits allowing concealed weapons, which faces a close vote; and a GOP substitute for the overall gun measure. The concealed weapons amendment, seen by advocates as protecting gun rights, was vehemently opposed by gun control groups, who say it would allow more guns into states with stricter firearms laws.

The other amendments were defeated.

The votes were coming a day after former Rep. Gabrielle Giffords, badly injured in a 2011 mass shooting in Tucson, Ariz., and her husband, Mark Kelly, tried galvanizing gun control support by visiting Capitol Hill and attending a private lunch with Democratic senators. Reid, D-Nev., called the lunch — senators said it included emotional speeches from lawmakers — “as moving as any” he has attended.

Lawmakers have touted the legislation expected to pass the General Assembly on Wednesday as the toughest in the country. Some measures would take effect right away, including the expansion of the state’s assault weapons ban, universal background checks for all firearms sales, and a ban on the sale or purchase of ammunition magazines holding more than 10 rounds. The bill also addresses mental health and school security measures in response to the massacre.

Gov. Dannel P. Malloy, a Democrat, supports the bill and could sign it into law as soon as Wednesday night.

In a state with a rich history of gun manufacturing, some companies said they feel the legislation made them into scapegoats for the deaths of 20 first-graders and six educators in the Dec. 14 massacre at Sandy Hook Elementary School. At least one ammunition magazine maker is more seriously considering offers to leave.

“My home is in Connecticut, but at this point, I don’t know if I can operate here legally come Wednesday afternoon,” said Jonathan Scalise, owner of Ammunition Storage Components in New Britain. He said it’s unclear to him whether employees in possession of banned firearms or ammunition would be breaking the law.

Gun shops across the state reported brisk sales Tuesday and said customers also checked on the status of orders that they worried could be canceled once the new laws take effect.

The parking lot at Hoffman’s Gun Center and Indoor Range in Newington was full Tuesday morning, with some drivers parking on the front lawn. Inside, customers waited in long lines to purchase what was left.

“I walked through. I walked out because they didn’t have anything. The girl told me what’s on the shelf is what they have. And I totally believe that,” said Nick Viccione, a gun owner from Wallingford. He said people are trying to load up on ammunition and buy “anything semi-automatic.”

At other shops, including the Delta Arsenal gun store in Wallingford, employees reported difficulty getting through to the state police to run background checks needed to complete gun sales.

Connecticut State Police Lt. J. Paul Vance said he hadn’t received any complaints of people not getting through to the office that handles such checks but acknowledged they might get a busy signal once in a while.

“The special licensing and firearms unit is going full bore,” he said.

The gun industry in Connecticut dates back to the Revolutionary War and says it supports more than 7,000 jobs in the state. Some companies say the new restrictions have them considering a move.

O.F. Mossberg & Sons Inc. in North Haven does not support a ban on firearms or equipment, said Joe Bartozzi, senior vice president and general counsel, adding that “they’ve never reduced crime or violence.”

Mossberg has been in business since 1919 and employs 270 workers in Connecticut. It also has a manufacturing plant in Eagle Pass, Texas, and has been courted for years by other states.

“I’ve got a stack of invitations from governors, congressmen and economic development groups right here on my desk,” he said.

Mark Malkowski, owner and president of Stag Arms in New Britain, said he’s not threatening to move but that his biggest concern about staying in Connecticut is “staying in a state that does not support us.”

He said Stag Arms, which employs 200 people, manufactures about 72,000 rifles a year, at a cost of about $1,000 each. About 5 percent of the company’s sales are in Connecticut.

“It’s actually quite hypocritical, quite insulting,” he said. “If our products are so dangerous and so horrible that no Connecticut resident should be in possession of it, why is it OK to send it to the rest of the country?”

Echoing that was the National Shooting Sports Foundation.

“Connecticut’s firearms manufacturers will be forced to pay a price economically for the state’s double standard of you can build it here, but not sell it here, public policy formulation,” the Newtown-based trade association said in a statement.

Malloy said Tuesday he plans to write to the state’s gun manufacturers, informing them that as long as they are manufacturing a product that can be legally sold in the U.S. that they’re still welcome in Connecticut.

“Having said that, there are other things that need to be taken into consideration,” Malloy said. “And the public’s safety is one of those things.”

Nicole Hockley, whose 6-year-old son, Dylan, was killed at Sandy Hook, welcomed the legislation. She credited lawmakers with listening to the parents, who had opposed allowing existing high-capacity magazines to be grandfathered into the law. Legislators did allow that but also required the magazines to be registered by Jan. 1 with the state.

“This is going to be one of the strongest gun laws in the nation, and that will be a model for other states to follow and for federal leaders to follow,” she said.

The president, in a set of brief remarks from the White House Thursday surrounded by the mothers of shooting victims, raised concern that the shock from the Newtown elementary school shooting could soon fade.

“Less than 100 days ago that happened. … Shame on us if we’ve forgotten,” Obama said. “I haven’t forgotten those kids. Shame on us if we’ve forgotten.”

Amid signals from Washington that the Senate bill could be losing momentum and high-dollar ad campaigns on both sides, the president was there to deliver a message to wavering lawmakers. “Don’t get squishy,” he said.

Obama, accusing opponents of drumming up “fear,” urged supporters to call members of Congress and pressure them into backing the package.

Republican Sen. Mike Lee, of Utah, ripped the president for the remarks, suggesting he was exploiting the Newtown tragedy.

“The proposals the president is calling for Congress to pass would primarily serve to reduce the constitutionally protected rights of law-abiding citizens while having little or no effect on violent crime,” Lee said in a statement. “It is deeply unfortunate that he continues to use the tragedy at Newtown as a backdrop for pushing legislation that would have done nothing to prevent that horrible crime.”

Lee said he and his conservative colleagues plan to ensure that any of the firearms proposals require a 60-vote threshold in order to proceed. This could be a high hurdle for the Senate to clear.

Already, Senate Majority Leader Harry Reid dropped from the main package a proposal to renew and expand the expired assault weapons ban.

The base bill will instead cover universal background checks, strengthened punishments for illegal trafficking and more money for school security.

Items like school security enjoy broad support. But the move to include private sales — including gun show purchases — in the background check system remains controversial in some circles. Plus the assault weapons ban, which Republicans most adamantly oppose, is still expected to get a vote as an amendment to the main bill.

The National Rifle Association, during and after Obama’s remarks, tweeted a flurry of comments criticizing the latest Obama push. The group accused him of campaigning “against guns to please the fringe.”

Both sides of the gun debate are stepping up their effort to sway Congress. While the NRA has been vocal since the start of the process, New York Mayor Michael Bloomberg just launched a $12 million ad buy in support of the legislation.

“It’s not done until it’s done,” Obama said Thursday. “There are some powerful voices on the other side that are interested in running out the clock, or changing the subject. … They’re doing everything they can to make all our progress collapse under the weight of fear and frustration.”